skin-deep

skin-deep  {adj.}
Only on the surface; not having any deep or honest meaning; not really or closely connected with what it seems to belong to.
Mary's friendliness with Joan is only skin-deep.
Ralph crammed for the test and got a good grade, but his knowledge of the lesson is only skin-deep.
Contrast: BRED IN THE BONE.
Categories: adjective
Source: A Dictionary of American Idioms

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